Resonance detector and dynamic tuner

Hi all, I'm working on a project where a standing resonance is a must.

I've already made many parts of my project and now I'm in the phase of the tuning.

What I need is a schematic which can accomplish two needs:

1. to sense/detect the resonant state of both sides of a coil, made of a primary and a secondary side - so a resonance detector
2. to adapt/adjust the capacitance (or the inductance) of the secondary side of my coil when a variable load is connected (e.g. an inductive load) - so a dynamic resonance tuner

In the field of the radio transmission this is a common function but, in my case, I have to deal with the power and not with a signal: the most difficult thing, for me, is that I have to manage high voltage spikes (10-30 kV) and, sometimes, with valuable amperage (5-20 A).

Let me give a simple example: I have a transformer (for the sake of simplicity) which I make it to oscillate, on the primary side, with a function generator, square wave, 35 kHz (there is a driver and a mosfet in the middle between the fgen and the transformer, but this is not important now); I've found the correct capacitance to get both the primary side and secondary side to resonate but, once an inductive load is connected on the secondary side, an imbalance occurs and the resonance is lost... of course I can adjust that imbalance using a new capacitor's value, accordingly with my load but...unfortunately, the load is not always the same: sometimes I have one kind of load, some other times there is another kind of load.

The kind of load, in my application, is unpredictable: in some cases it is not inductive but, instead, capacitive or resistive.

So I thought to the varicap (varactor) which can be voltage-controlled by a micro (Arduino in my case) but these kind of components have a so low capacitance values (in the order of few pF) that this makes them useless for my purposes. Furthermore, in the presence of so high voltage peaks, they will be destroyed in microseconds!

Could someone give a viable solution?

Many, many thanks in advance

There's no chance to use electronic components in the high kV range, except for vacuum tubes. Most likely you'll end up with mechanical or electro-mechanical tuning means.

To make your better understandable, you should describe the transformer parameters in terms of self and mutual inductance, respectively winding ratio and coupling factor.

For auto tuning short wave transmitters they check the phase of the current against the voltage in the tank circuit, when they are in phase its tuned! The normal tuning would be done by motor driven coils/capacitors.
I wonder if a saturable inductor would be the way to get some sort of reactance control at your power levels.
Frank

V/I phase is probably the way to do it but the values of tuning elements at 35KHz would probably make it impractical to build.

Brian.

chuckey said:

For auto tuning short wave transmitters they check the phase of the current against the voltage in the tank circuit, when they are in phase its tuned! The normal tuning would be done by motor driven coils/capacitors.
I wonder if a saturable inductor would be the way to get some sort of reactance control at your power levels.
Frank

Click to expand...

Hi Frank, thank you so much for your answer, you enlightened me: I can make use of a saturable reactor!

You're a genius!

Do you know the theory of operation of a magnetic amplifier? If you're curious here it is:
https://en.wikipedia.org/wiki/Magnetic_amplifier

With such "old tech" device I can drive any power and any voltage (even my spikes with tens of kV) simply applying a small control voltage to saturate the small trasformer-driver.

Any idea (preferably with a schematic ;-) ) on how to detect the resonance's state?

I thought of a solution like this:


What do you think?

If you convert the V and I samples into square waves then applying them to the input of an exclusive OR gate, will give you a mean DC level, varying from 0V when the square waves are in phase to +Vcc when the are anti phase..
Frank

This sounds similar to the classic induction heater problem.
As soon as you introduce a load (into) the tuned tank this upsets the tank tuning causing a serious power loss.

This is always likely to be a problem where an oscillator drives a power amplifier which excites the tuned tank.

The trick seems to be to make the resonant tank a part of the actual oscillator circuit, so as the tuning changes with a varying reactive load, its still remains at resonance, but the frequency will have moved.

It will still usually couple power optimally into the load because its still at resonance.
That may help with your particular problem, or it may not ?

But one way to do this is to drive your amplifier and tank from a phase locked loop which locks to itself via a small energy sample from the output tank.

You can still use some type of tuning, a saturable reactor, motor driven capacitor, or whatever, to pull it back on frequency if fixed frequency operation is a requirement.